Semiconductor devices implemented by integrated circuits are widely used for various electronic applications or communications systems. In the transmission of information or data via electrical signals, transmitting as much of the power as possible from power source to load is often desirable. However, undesired variations in behavior of semiconductor devices resulting from manufacturing processes may result in impedance mismatches, which may cause large power consumption and limit the accuracy of the circuit behavior. Accordingly, it is typically desirable to reduce the impact of impedance mismatches on the performance of circuits and to maximize the power transfer. There are a variety of techniques to reduce the impact of impedance mismatches. Among them, impedance matching is the most commonly used technique. Impedance matching may involve attempts to make an output impedance of a source, such as a power amplifier (PA) of a communications system, and an input impedance of a load, such as an antenna or a wired communications medium, attain a desired relationship so that maximum power transfer, maximum voltage transfer, maximum efficiency, minimum signal reflections, and so forth, are achieved.
There are various devices, for example, termination resistors, transformers, inductors and capacitors or combination of them, that may be used between power source and load that perform impedance matching. In many conventional impedance matching circuits, termination resistors may reside outside the semiconductor device or may be integrated into the circuit boards to lower the cost. In the later example, a trimming circuit is by far the most common technique to more precisely match source impedance to load impedance.
FIG. 1 is a schematic diagram of a conventional impedance matching circuit 100 utilizing a trimming circuit. As is shown, the circuit 100 includes an amplifier circuit 102 and a trimming circuit 104. The trimming circuit 104 includes a termination resistor RS0 and a plurality of trimming resistors RS1, RS2 . . . RSN in parallel. The trimming resistors are coupled to respective metal-oxide-semiconductor (MOS) transistors S1, S2, . . . SN used as switches in series. For example, the trimming resistor RSN is coupled to the MOS transistor SN in series. Because the inherent nonlinear characteristics of the MOS transistors are not included in the negative feedback circuit, the MOS transistors may transfer the nonlinearities through the trimming resistors to the load, and hence cause distortions of the input signals which may degrade the linearity of the output signal Vout.